S-IC

• 1. Introduction

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  Five F-1 engines at the base of the S-IC generated 7,750,000 pounds of thrust at sea level. The S-IC was the first stage of the Saturn V rocket. More than 90 percent of its launch mass was propellant: RP-1 rocket fuel and liquid oxygen. Standing 138 feet tall and 33 feet in diameter, the stage had to push everything above it through the first 61 kilometers of atmosphere. How did engineers manage the ignition of engines this powerful on a single structure? What did it take to manufacture these machines, and what became of each one once it had done its job?

• 2. Building at Michoud

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  On the 15th of December 1961, Boeing was awarded the contract to manufacture the S-IC stage. The general design had already been settled by engineers at Marshall Space Flight Center before Boeing came aboard. The primary site of manufacture was the Michoud Assembly Facility in New Orleans. Wind tunnel testing took place in Seattle. The specialized machining of the tools needed to build the stages was done in Wichita, Kansas.

  Marshall Space Flight Center itself built the first two flight stages, S-IC-1 and S-IC-2, along with two test versions. Building the propellant tanks alone took seven to nine months. Completing a full stage required 14 months from start to finish. Boeing's first unit was S-IC-D, a ground test dynamics model, followed by an additional test stage designated S-IC-F.

  NASA initially ordered 15 flight stages to support the Apollo program. In July 1967, Boeing received a contract to begin acquiring long-lead-time components for a 16th and 17th stage. A full contract for S-IC-16 through S-IC-25 was drafted during the middle of 1967. Every stage beyond S-IC-15 was canceled in October of that year due to budgetary restrictions. Boeing's ground dynamics test model, S-IC-D, is preserved today at the U.S. Space and Rocket Center in Huntsville, Alabama.

• 3. The Ignition Sequence

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  Three hundred milliseconds separated each of the three S-IC engine ignition events. The center engine fired first. Then a diagonal pair of outer engines lit. The remaining two followed last. Firing all five at once would have placed severe stress on the thrust structure below. The staggered approach kept those loads manageable.

  The center engine was fixed in place and could not pivot. The four outer engines were mounted on hydraulic gimbals, which is how the Saturn V was steered during ascent. Together they sat in a quincunx arrangement, the same pattern as the five dots on a die.

  At 48,000 pounds, the thrust structure was the heaviest single component of the S-IC. It absorbed the combined force of the engines and redistributed it evenly across the rocket's base. Four hold-down anchors secured the vehicle to the pad while it built to full thrust before release. These were among the largest aluminum forgings produced in the United States at the time, each 15 feet long and weighing 1,800 pounds. The four stabilizing fins endured temperatures reaching 2,000 degrees Fahrenheit during ascent. The propellant tanks above, one for fuel and one for liquid oxygen, each brought their own distinct engineering demands.

• 4. Tanks and Separation

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  The liquid oxygen tank held 334,500 US gallons and posed a problem unlike any other part of the stage. The feed lines carrying LOX to the engines had to run completely straight. Any bend would slow the flow, requiring larger and heavier piping. The only viable path was directly through the fuel tank below. Engineers enclosed each line in an insulating tunnel to prevent the extreme cold of the liquid oxygen from freezing the surrounding fuel. Five extra holes had to be cut into the top of the fuel tank to accommodate these penetrations.

  Below the LOX tank, the fuel tank held 209,000 US gallons of RP-1. At a dry mass of more than 12 short tons, it could release propellant at 1,300 US gallons per second. Before launch, nitrogen was bubbled through the fuel to keep it properly mixed. During flight, helium pressurized the fuel; that helium was stored in tanks mounted inside the LOX tank above. Both the fuel tank and the thrust structure were painted in alternating black and white bands. This pattern let engineers monitor the vehicle's roll during ascent.

  Between the two propellant tanks sat the intertank. It housed fill and drain lines for the LOX tank and a section of the five liquid oxygen feed lines running toward the engines. The forward skirt, mounted atop the LOX tank, connected the S-IC to the S-II stage and contained telemetry equipment and vent lines.

  Eight solid retrorocket motors, two inside each of the four conical engine fairings, fired at stage separation. They blew off removable sections of the fairings and pushed the spent stage back away from the accelerating vehicle above. The propellant tanks were made from 2219-series aluminum panels. The forward skirt, interstage, and thrust structure used 7075-series aluminum, corrugated with external stringers for added rigidity. The stage also carried an ODOP transponder, which let ground controllers track the vehicle's trajectory after liftoff. The first stage built to this complete specification flew before the end of 1967.

• 5. From Apollo to Skylab

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  S-IC-1 lifted off on the 9th of November 1967 on Apollo 4, the first flight of the Saturn V. S-IC-2 followed on the 4th of April 1968 on Apollo 6. That stage was fitted with television cameras on its boattail and forward skirt.

  S-IC-3 flew on Apollo 8 on the 21st of December 1968. It was the first flight stage produced by Boeing rather than MSFC, and it was lighter than both predecessors. That weight reduction allowed the mission to carry an additional 36 kilograms of payload. S-IC-5 launched Apollo 10 on the 18th of May 1969 and was the last flight stage to carry full research and development instrumentation.

  S-IC-6 lifted Apollo 11 on the 16th of July 1969. Decades later, a team financed by Jeff Bezos recovered one or more of its F-1 engines. S-IC-10 launched Apollo 15 on the 26th of July 1971 with only four retrorockets instead of the standard eight. They were mounted in two diagonally opposite compartments. Later missions returned to the full complement of eight.

  S-IC-13 carried Skylab 1 on the 14th of May 1973, the final S-IC flight. For that mission, the engine shutoff sequence changed from the standard 1-4 pattern to a 1-2-2 sequence. This reduced structural loads on the Apollo Telescope Mount.

  S-IC-14 and S-IC-15 were completed but never flew. S-IC-14 had been assigned to Apollo 18, planned for 1974, and is now on display at Johnson Space Center. S-IC-15 was intended for Apollo 19, also planned for 1974, and was at one point considered a possible Skylab backup. It remained at the Michoud Assembly Facility until June 2016, then moved to the INFINITY Space Center in Mississippi.

  The test stage S-IC-T, nicknamed "T-Bird" and assembled between 1963 and 1965, completed at least 22 test firings between 1965 and 1967. It now stands as part of the Saturn V exhibit at Kennedy Space Center.

• 6. Studies and Alternatives

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  A 1960 study proposed an S-IC variant with eight F-1 engines for a vehicle called the Saturn C-8. Three separate 1961 studies each proposed five-engine variants for early Saturn configurations, including the C-3B, C-4B, and C-5. A 1965 study called the Flat Bulkhead variant examined reducing the stage's length and structural weight.

  A 1967 concept designated the Saturn S-IC-TLB proposed a two-engine reusable booster. That same year, a "stage and a half" configuration study described a single-engine sustainer design. This sustainer was intended to remain powered longer than a conventional first stage would. In 1968, engineers studied a four-engine recoverable booster called the S-ID Booster. It was aimed at a proposed rocket family designated the Saturn V-B, V-C, and V-D. A companion single-engine sustainer was studied alongside it for the same vehicle family. None of these configurations reached hardware. The S-IC-TLB reusable concept appeared the same year NASA was canceling stages beyond the 15th. While the production line was closing, proposals for new forms of the stage were still being written.